Making sure each routing point has at least two ways to get to/from it seems to help quite a bit.  Imagine an eight-lane highway where for a mile, all traffic both ways has to go through only one lane.  If you only have one routing dot connecting two areas, you get the same sorts of traffic issues in-game. This sort of pileup was common in front of schools as many sim children would show up at once and wait ages to be routed through the door.  Definitely not something you want on every sim's daily commute!